P0301 — Fault Tree

P0301 — Cylinder 1 misfire detected

One cylinder is not burning its fuel properly. Unburnt fuel goes into the exhaust and can destroy the catalytic converter in a few hundred miles, which turns a cheap repair into an expensive one.

System: Powertrain
Definition: Generic (SAE-defined, all manufacturers)
Subsystem: Ignition system or misfire
Severity: severe

Causes, by prior probability

Cause	Prior	Repair
Ignition coil, cylinder 1 Swap the coil to another cylinder. If the misfire follows, the coil is the fault.	34%	$135
Spark plug, cylinder 1 Pull and inspect. Check gap and insulator colour against the others.	28%	$85
Fuel injector, cylinder 1 Listen with a stethoscope for a clicking injector; measure coil resistance.	16%	$344
Low compression, cylinder 1 Compression and leak-down test. This is the diagnosis that changes the economics of the whole repair.	13%	$213
Vacuum leak local to cylinder 1 Smoke test with attention to the runner for cylinder 1.	9%	$180

These are priors from the code alone. A scan's freeze frame reorders them, which is the entire point of capturing it.